In the brushless DC motor (BLDC) drive circuit, a bootstrap capacitance calculation method based on energy conservation is proposed to solve the problem that the gate drive voltage of power field effect transistor (MOSFET) fluctuates greatly. According to the BLDC dynamics model, the single commutation time interval in the six-step commutation is analyzed. The equivalent circuit of BLDC and MOSFET is used to construct the motor drive circuit model, and the energy transfer direction in the circuit is analyzed to calculate the bootstrapping capacitance value. According to the method proposed in this paper, a brushless DC motor drive system is built, and the peak valley difference of the grid driving voltage of the circuit is 0.2V. The experimental results show that, compared with the traditional empirical method, the capacitance selected by this method can improve the grid driving voltage stability by 80%.